Using solar energy to replace fossil energy is becoming increasingly important and is gaining popularity to save the environment of earth and to have a sustainable energy resource for human kind. Usually many solar cells (which are the semiconductor devices that generate electric current under light) are connected in series to form a solar panel, and many solar panels are connected in series to form a solar panel array. An inverter (also called “central inverter”) is usually connected to the solar panel array to convert the power from the solar panel array to a desired voltage or current. The key to facilitate the solar energy usage adaptation is to make solar energy high efficiency and low cost. The low cost refers to not only the low cost of solar cell itself, but also the low cost of solar panel array installation, testing, monitoring and debugging.
Since a solar panel array can have a high voltage of several hundred volts, the safety is a big concern during solar panel installation and maintenance. For example, in order to provide the firefighters' a safe access to the roof with solar panel installation, the State of California passed a new law, effective from Jan. 1, 2014, which mandates at least 3-feet setback from the roof ridge line for solar panel installation in all California cities and counties. This new setback guidelines make the ideal solar roof space unavailable for solar panel installation, and will increase the overall cost of solar power system. Another serious safety concern about the solar panel is the arc fault. Arc faults are caused by air gaps between electrical conductors associated with the solar panel system. The most common causes are faulty manufacturing, installer error, or aging, degraded connectors. When an arc fault happens, the electrical current across the loose connection produces sparks which could cause a fire. The 2011 National Electrical Code (NEC) requires the rooftop solar arrays to equip an arc fault current interrupter (AFCI). The AFCI currently available on market are not only expensive, but also prone to nuisance-tripping in response to other electrical activities that are not in fact arc faults.
The high efficiency of solar power system depends on not only the high energy conversion efficiency of solar cell itself, but also how effectively the available solar cell power can be extracted out and converted into useful form of energy. Solar cell has its unique voltage-current curve which results in a Maximum Power Point (MPP) where the solar cell can output maximum power. In practice the solar cells are usually connected in series to form a solar panel, and many solar panels are connected in series to form a solar panel array. Serial connection overall brings lower installation cost and higher inverter's efficiency, but it also has a serious problem: the current of a string of solar cells is limited by the weakest cell in the string. So if one solar cell in a string is shaded or severely degraded, it will drag down the output power of the whole string dramatically. In this situation, the rest of the cells in the string will generate high reverse bias voltage across the shaded cell and will break it down and cause irrecoverable damage (this is usually called “hot spot” problem). To address this issue, people usually add a bypass diode to every solar panel, to bypass the underperforming solar panel. If a small number of solar panels in a big solar panel array are shaded, the total output power of the solar panel array may have multiple local maximum points instead of one. The central inverter can be stuck in a local maximum power point and fail to find the global and best maximum power point. This will result in much lower efficiency of solar panel array since its best maximum power is not extracted out by the central inverter. A solar panel array with multiple local maximum power points brings the central inverter a fundamental problem: if the inverter's Maximum Power Point Tracking (MPPT) algorithm searches narrowly on power-voltage curve of the solar panel array, chances are that it will be stuck in a local maximum power point; but if the inverter's MPPT algorithm searches widely, the inverter will deviate far from the best maximal power point. In other words, the normal operation of the solar panel array will be interrupted during the search.
To solve or alleviate above problems, some companies proposed to divide the solar panel array into multiple sub-arrays or panels and use a micro-inverter for each panel. Each micro-inverter converts the power from the panel it connects to and then sums the power from all micro-inverters together. This approach needs many expensive micro-inverters and more complicated routing, so its cost is prohibitively high.
A solution to the above problems is highly desired, to increase the safety of solar panel array, increase the overall efficiency of solar power system, and to lower the solar panel installation cost.